Recent surveys indicate that between 10 and 20 percent of homes in the United States have indoor radon levels that exceed EPA's guideline level for remedial action, resulting in an estimated 5,000 to 20,000 lung cancer deaths each year. This estimate is based on extrapolating from results of studies of miners with very high radon exposures; findings from these studies may not be generalizable to the population at large. The relationship between cumulative residential exposure to radon and cancer risk is being evaluated in a collaborative study involving Yale University and the University of Utah. The study will include 1000 smokers with lung cancer, 750 nonsmokers with lung cancer, and over 2100 population controls from Connecticut, Utah, and Southern Idaho. Because smoking may enhance the effects of radon exposure, the study is designed to evaluate the potential interaction between radon and cigarette smoke exposure. Detailed residential and exposure histories are obtained through the use of computer assisted telephone interviews (CATI). Radon measurements are made in past homes using year-long alpha track etch detectors, in order to estimate cumulative radon exposure since age 25 for each subject. Complete lifetime exposure assessments (including childhood) will be made for a subset of participants. A companion study in Connecticut will evaluate the potential childhood cancer risk associated with residential radon exposure. Cumulative radon exposure will be determined for approximately 125 childhood cancer cases and 250 healthy comparison subjects. Standardized data collection instruments have been developed, and case and control identification has begun. Preliminary plans for modeling radon exposures have been formulated.